1. Field of the Invention
This invention relates to an optical device for distributing light from a light source or a light emitting body, and more particularly, to a sheet type optical device for diverging the light in a required direction. Although this invention is useful for a variety of different applications, it is particularly useful for a backlighting unit employing the optical device adapted for use in a liquid crystal display device or the like.
2. Description of the Related Art
In a lighting panel for a liquid crystal display device or the like, a sheet type optical device is typically used to increase the light quantity in the sheet by concentrating the transmitted light in a specified angular range or by concentrating a light in a specified direction. An example of such a sheet type optical device includes a lighting panel disclosed in U.S. Pat. No. 4,542,449 issued to Lorne A. Whitehead on Sep. 17, 1985. The lighting panel comprises two sheets made of a transmissive material, called "prism sheets," each having a smooth surface and a corrugated surface in such a manner that light incident obliquely onto the sheet is emitted in a direction perpendicular to the sheet. In the lighting panel comprising two such sheets, the emitted light becomes uniform in the entire surface of the panel and the light quantity increases.
On the other hand, a backlighting unit used in a liquid crystal display device or the like provides a high luminance in a specified angular direction and concentrates the emitted light in a specified direction to increase the light quantity. To this end, the backlighting unit must use a sheet type optical device that allows a transmitted light to be oriented (or concentrated) in a specific direction. Accordingly, in the conventional backlighting unit, a lighting panel comprising two prism sheets as described above is used. As shown in FIG. 1, such a backlighting unit includes a lamp 10 for generating light, a light guide 12 for guiding the light from the lamp 10, and a lamp housing 14 installed at the side surface of the light guide 12 in a curved shape partially surrounding the lamp 10. A cold cathode ray tube is typically used as the lamp 10. The light generated at the lamp 10 is received through the side surface of the lighting guide 12. The lamp housing 14 includes a lamp reflector provided at the inner surface thereof. This lamp reflector enhances the utilization ratio of the light generated by the lamp 10 by reflecting the light from the lamp 10 toward the side surface of the light guide 12. The light guide 12 formed of a transparent material including plastic such as PMMA. The light guide 12 may have an inclined lower surface and a horizontal upper surface, or, alternatively, an inclined upper surface and a horizontal lower surface. The light from the lamp 10 travels through the panel and exits upwardly from the upper surface. A reflective plate 16 is provided at the lower side of the light guide 12, and a diffusion panel 18, first and second prism sheets 20 and 22 and a protective film 24 are sequentially arranged over the upper side of the light guide 12. The reflective plate 16 serves to reduce light loss by reflecting light transmitted downwardly through the lower surface of the light guide 12 back toward the light guide 12. The reflective plate 16 also improves the uniformity of the transmitted light across the upper surface of the light guide 12. The light guide 12 cooperates with the reflective plate 16 in this manner to guide the light from the lamp 10 upwardly. The light exiting the upper surface of the light guide 12 travels in a direction within an angular range from the normal direction of the upper surface.
The diffusion panel 18 positioned between the light guide 12 and the first prism sheet 20 distributes the light received from the light guide 12 and prevents light spots from forming due to partial concentration of the light. The diffusion panel 18 may also bend the light beams upwardly to a direction closer to the normal direction of the sheet. The first prism sheet 20 and the second prism sheet 22 constitute the above-mentioned lighting panel. The first prism sheet 20 has a smooth lower surface and an upper surface that is corrugated in the transverse direction (the direction perpendicular to the axis of the light source 10); while the second prism sheet 22 has a smooth lower surface and an upper surface that is corrugated in the longitudinal direction (the direction parallel to the axis of the light source 10). The first prism sheet 20 converges the light traveling toward the second prism sheet 22 in the transverse direction, and the second prism sheet 22 converges the light traveling toward the protective film 24 in the longitudinal direction. Because of the converging function of the first and second prism sheets 20 and 22, the direction of the light from the diffusion panel 18 is changed in such a manner that it travels toward the protective film 24 in a direction substantially perpendicular to the sheets. Accordingly, the light passing through the first and second prism sheets 20 and 22 travels almost perpendicularly to distribute uniformly on the entire surface of the protective film 24. Finally, the protective film 24 is used to protect the surface of the second prism sheet 22. It may also serve to diff-use the light in order to generate a more uniform distribution of the light.
As described above, the conventional sheet type optical device such as the lighting panel described above must use two prism sheets to generate a beam of light emitting from the panel at an angle of constant angular range. Thus, the conventional sheet type optical devices have a complicated structure, require increased number of steps in the fabricating process, and result in increased manufacturing cost. Further, for a backlighting unit using the two prism sheet structure, it is difficult to reduce the thickness of the panel to below a certain limit. Thus, such a backlighting unit suffers from the complex structure and increased manufacturing costs.